Genetic analysis of cell-cell signaling in Saccharomyces cerevisiae has led to the identification of a novel factor, known as Sst2p, that promotes recovery after pheromone-induced growth arrest (R. K. Chan and C. A. Otte, Mol. Cell. Biol. 2:11-20, 1982). Loss-of-function mutations lead to increased pheromone sensitivity, but this phenotype is partially suppressed by overexpression of the G protein alpha subunit gene (GPA1). Suppression is allele specific, however, suggesting that there is direct interaction between the two gene products. To test this model directly, we isolated and characterized several dominant gain-of-function mutants of SST2. These mutations block the normal pheromone response, including a loss of pheromone-stimulated gene transcription, cell cycle growth arrest, and G protein myristoylation. Although the SST2 mutations confer a pheromone-resistant phenotype, they do not prevent downstream activation by overexpression of G beta (STE4), a constitutively active G beta mutation (STE4Hpl), or a disruption of GPA1. None of the SST2 alleles affects the expression or stability of G alpha. These results point to the G protein alpha subunit as being the direct target of Sst2p action and underscore the importance of this novel desensitization factor in G-protein-mediated signaling.